Future generations of wireless system are expected to provide ubiquitous high data-rate coverage. Achieving this requires an efficient use of the available resources. In light of this, the use of more advanced and adaptive antenna has been given an increased attention. For example, by using multiple antennas at the transmitter and/or the receiver, it is possible to exploit the spatial degrees of freedom offered by the multipath fading inside the wireless channel in order to provide a substantial increase in the data rates and reliability of wireless transmission. Focusing the transmitted energy in certain directions is usually called beamforming.
Cell shaping and UE-specific beamforming are two types of beamforming techniques that significantly can improve performance in cellular networks. Both these techniques can be implemented by using antenna arrays at the base station (BS). Another technique that utilizes the spatial spread of the users and the wireless channel is sectorization. Sectorization is a matter of splitting one cell into two or more, and has traditionally been done by adding hardware, for example replacing one antenna (and RBS) by two new. With the use of advanced antennas, it is however possible to create two sectors (instead of one) from the same antenna. This is typically referred to as sectorization with aperture reuse. One of the major advantages of introducing sectorization in wireless systems is a potential improvement in capacity. More cells allow for more simultaneously scheduled users. Basically, the spatial structure of the channel (and spread of users) is exploited to be able to reuse the resources within a given area.
A conventional way to do sectorization is to divide a site in a number of horizontal sectors, with three sectors being most common. In some high-traffic areas also six horizontal sectors per site are deployed. Another way to increase the sectorization is to split each horizontal sector into two vertical sectors, called vertical sectorization. Recently, there has been a growing interest in vertical sectorization. One problem with vertical sectorization in flat scenarios is that the load balance between the upper and lower cell typically becomes rather uneven. One reason for this is that the user distribution typically is unevenly distributed w.r.t. the elevation angle, where there is typically a large concentration of users just below the horizon. When creating two vertical sectors, the sector that covers the user close to the horizon typically gets much higher traffic load than the other sector, which typically is not optimal for the system performance. One way to mitigate this problem while maintaining high antenna gain to served users is to create one narrow beam that covers the users close to the horizon and one wider beam that covers the other users.
Aperture reused vertical sectorization with the flexibility of changing the beam widths for respective sector beam can typically be done with an active antenna with radio behind each antenna element. One problem with this solution is that it requires many radios (TRXs) which make the product costly to design and manufacture.